1. Field of the Invention
The present invention relates to heat resistive phosphate-zircon composite bodies comprising a solid-solved phase having a composition of R.sub.y Zr.sub.4 Si.sub.x P.sub.6-x O.sub.24 and a zircon phase. In the above formula, R consists of one or more kinds of elements capable of becoming bivalent or trivalent cations, x is a value less than 2, and y is a value satisfying an electrically neutral condition and greater than 2/3 but less than 2. The invention also relates to a process for producing the same. More particularly, the present invention relates to heat resistive phosphate-zircon composite bodies having excellent high temperature thermal stability and strength and composed of the solid solved phase having the composition of R.sub.y Zr.sub.4 Si.sub.x P.sub.6-x O.sub.24 having excellent heat resistance and high temperature stability and the zircon phase. The invention also relates to a process for producing the same.
2. Related Art Statement
Recently, demands for materials having excellent heat resistance and low expansion have been increasing with developments in industrial technology.
Under these demands, it has been made clear that zirconyl phosphate [(ZrO).sub.2 P.sub.2 O.sub.7 ] is promising as a material having excellent heat resistance and low expansion.
Further, zirconium phosphates of alkali metals such as sodium have been recently proposed as materials having heat resistance and low coefficients of thermal expansion [Mat. Res. Bull., Vol. 19, pp 1451-1456 (1984), Journal of Materials Science 16, 1633-1642 (1981), and Yougyou Kyokaishi, 95[5], pp 531-537 (1987)).
Furthermore, phosphate compounds of alkaline earth metals having special compositions have bee proposed as having low expansion (Mat. Res. Bull., Vol. 20, pp 99-106, 1985, J. Am. Ceram. Soc., 70[10]C-232 to C-236 (1987) and U.S. Pat. No. 4,801,566).
U.S. Pat. No. 4,675,302 proposes that ceramic materials having a fundamental composition of Ca.sub.0.5 Ti.sub.2 P.sub.3 O.sub.12 have excellent low expansion.
In addition, trials have been made to combine special phosphate compounds and zircon (Journal of Materials Science, Vol. 20, 4617-4623 (1985) and EP-0260893A2).
Moreover, EP-0306242A2 discloses a solid solved phase of R.sub.y Zr.sub.4 Si.sub.x P.sub.6-x O.sub.24, sintered bodies containing this solid solved phase, and a process for producing the same.
However, although the phosphate compounds such as zirconyl phosphate have the merit that they have the excellent low expansion, the compounds are thermally decomposed at high temperatures more than 1,200.degree. C. so that a phosphorus (S) component evaporates. For example, there is a problem that when thermally treated at 1,400.degree. C. for 100 hours, zirconyl phosphate and zirconium sodium phosphate exhibit weight reductions of as much as 19% and 36%, respectively.
The ceramic materials proposed in U.S. Pat. No. 4,675,302 are used mainly as substrates for low expansion optical reflectors to be used in artificial satellites and undergoing no deformation or the like even due to changes in temperature. As shown in FIG. 2 of this publication, the temperature change is considered about 500.degree. C. at the maximum. Thus, no consideration is paid at all with respect to stability and heat resistance at high temperatures, for example, not less than 1,200.degree. C.
As a process for producing phosphate compounds, a process using a combination of Na.sub.2 CO.sub.3, ZrO.sub.2, ZrOC.lambda..sub.2. 8H.sub.2 O, SiO.sub.2, (NH.sub.4).sub.2 HPO.sub.4, H.sub.3 PO.sub.4, Nb.sub.2 O.sub.5, Y.sub.2 O.sub.3, Sr CO.sub.3, K.sub.2 CO.sub.3, and CaCO.sub.3 is known [T. Oota and I. Yamai, Journal of the American Ceramic Society, 69, 1 (1986)].
However, in the above process, a P.sub.2 O.sub.5 component is singly produced during decomposition of ammonium phosphate or H.sub.3 PO.sub.4, so that portions having high concentrations of phosphorus are locally formed, and a low melting point compound is formed during firing. Consequently, large pores (spaces) are formed around the low melting point compound in the sintered body, which causes great defects.